Need help any answers will help

So on May 1st I started my period it was normal. On May 7th I had unprotected sex. Then on May 19th I got my period again. It was a full length period with all symptoms. I went to the doctor he did a pregnancy test came out negative he tested me for all stds came up negative. I was having pain on my lower left side he couldn't find out why now I'm bleeding again it's just spotting but still why am I bleeding so much ? Anyone else had this problem?